TITLE:   GCN CIRCULAR
NUMBER:  32848
SUBJECT: GRB 221024A: Swift/UVOT UV upper limits
DATE:    22/10/25 14:54:44 GMT
FROM:    Paul Kuin at MSSL  <npkuin@gmail.com>

N. P. M. Kuin and R. Brivio report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB221024A
127 seconds after the BAT trigger (Brivio et al., GCN Circ. 32827).
The UVOT could not observe in the u,b,v or white filters due to the
nearby bright star nu uMa (V=3.49, U=6.44).  A source detection has
been reported by Strobl et al. (GCN Circ. 32838) and by Swain et al.
(GCN Circ. 32841) with r' = 18.36 at 9.12 hours after the Trigger.

UVOT obtained images in uvw1, uvm2 and uvw2. Visual inspection shows
no credible source in the initial images with upper limits of 19.7 mag.

In the uvm2 filter which has no red leak we obtained deeper limits:

filter   tstart(s)   tstop(s) exposure 3-sigma UL
uvm2     23992       36065      609      >21.3
uvm2     51885       52503     3306      >20.3

The limits in the UV are about 3 mag. below the reported detections
in r', which may suggest a redshift larger than 1.5.

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.05 in the direction of the burst
(Schlegel et al. 1998).